Process Dynamics:

What is the time constant of a first$-$order process?

A$)$ The time taken to reach $63\mathrm{.}2\%$ of the final value

B$)$ The time taken to reach $50\%$ of the final value

C$)$ The time taken to reach $90\%$ of the final value

D$)$ The time taken to reach $100\%$ of the final value